1. Field of the Invention
The present invention relates to an optical amplifier which supplies a pumping light to an optical amplification medium to amplify a signal light, and an optical transmission system using the same, and in particular, to a technology for controlling a supply condition of pumping light in the optical amplifier.
2. Description of the Related Art
Recently, there has been introduced a wavelength division multiplexing (WDM) technology for achieving a large capacity and a high speed in a trunk optical transmission system. Further, as a core technique of WDM transmission technology, an optical amplification technique, such as a rare-earth element doped fiber optical amplifier, a Raman amplifier and the like, has been in practical use.
FIG. 38 is a block diagram of an optical transmission system using typical optical amplifiers. In this system, a plurality of repeater stations are disposed between a transmission station (Tx) 1101 and a reception station (Rx) 1102, and a WDM light is transmitted via these repeater stations. In each repeater station, Raman amplification is performed. Further, each repeater station is provided with a discrete optical amplifier, such as, an erbium-doped fiber optical amplifier (EDFA).
A transmission path fiber 1001 is an optical transmission medium propagating the WDM light therethrough, and also functions as an optical amplification medium by being supplied with a pumping light. A pumping light source (LD) 1002, which is formed by multiplexing by a multiplexer or the like, for example, emission light from a laser diode or a plurality of laser diode, generates a pumping light for amplifying the WDM light. Here, the pumping light generated in the pumping light source 1002 contains a plurality of lights having wavelengths different from each other. A WDM coupler 1003 introduces the pumping light generated in the pumping light source 1002 to the transmission path fiber 1001.
In the above optical transmission system, the WDM light sent out from the transmission station 1101 is transmitted up to the reception station 1102 while being amplified by each transmission path fiber 1001. At this time, in each repeater station, the output power of the entire WDM light is managed, and also the balance of the optical powers of a plurality of signal lights contained in the WDM light is managed. Namely, the pumping light source 1002 is controlled so that, for example, the output power of the entire WDM light is maintained at a previously set predetermined value and the optical powers of the plurality of signal lights contained in the WDM light are equalized, in each repeater station (refer to Japanese Unexamined Patent Publication No. 2002-72262 (FIG. 3, 3 to 5 pages), Japanese Unexamined Patent Publication No. 2000-98433 (FIG. 1, paragraphs 0070 to 0072), and Japanese Unexamined Patent Publication No. 2002-76482 (FIG. 10, paragraphs 0162 to 0177)). Further, other than the output constant control or the control of wavelength dependence of gain as described above, a shutdown control at a signal light interruption is also performed by monitoring the output power of the WDM light. Note, the shutdown control is generally provided in the optical amplifier, as a function for, when a pumping light of high power is leaked to outside by the system crash, the optical fiber cutting and the like due to a surge, avoiding the radiation of the pumping light to a human body.
However, in the existing optical transmission system as described above, there is a problem in that it is difficult to accurately monitor the balance (optical power tilt) of the output powers of the plurality of signal lights contained in the WDM light. For example, in the above mentioned Japanese Unexamined Patent Publication No. 2002-72262, a signal light band is divided into a plurality of blocks, and a control of optical power tilt is performed utilizing the optical power detected for each block. However, in this case, when the signal lights are not arranged equally in each block, since the optical power tilt cannot be detected accurately, it is impossible to equalize the WDM light. Note, such a problem is not generated only in the system described in the above described Japanese Unexamined Patent Publication No. 2002-72262, but also generated in the case where the signal lights are arranged unevenly on a specific wavelength region in the signal light band, even if the optical powers of the plurality of signal light contained in the WDM light are detected individually.
Further, in the case where the output power of the entire WDM light is detected using a photodiode or the like, the photodiode receives lights over a wide band. Therefore, when the number of signal lights contained in the WDM light is small, a noise light caused by ASE (amplified spontaneous emission) or the like becomes dominant, (that is, a ratio of noise light power to the total optical power becomes relatively high). Therefore, there is also a problem in that the optical power of a main signal light (that is, the WDM light which is to transmit signals) cannot be detected accurately.
Here, there will be described in detail a monitoring value of signal light utilized for the control of pumping light as described above.
Generally, in the optical transmission system using the Raman amplifier as shown in FIG. 38 described above, for example, as shown in FIG. 39, since the noise light due to Raman amplification is generated within the signal light amplification band in the transmission path fiber being the optical amplification medium, a monitor of output signal light receives simultaneously the signal light containing noise components accumulated in the repeating intervals until the former stage, and the noise light due to Raman amplification. The above noise light due to Raman amplification is a noise light, which is also generated in the case where only the pumping light is input to the optical amplification medium in a state where the signal light is not input to the optical amplification medium. In this specification, the noise light generated in the Raman amplifier is called an amplified spontaneous Raman scattering (ASS) light, to an ASE light generated in the rare-earth element doped fiber amplifier, such as EDFA or the like.
As a conventional technique for monitoring the signal light output power of the Raman amplifier, for example, as shown in FIG. 40, there has been known a method of calculating the ASS optical power generated in the Raman amplifier based on the power of pumping light supplied to the optical amplification medium, to perform a correction by subtracting the ASS optical power from a monitoring value of an actually received output light (refer to the pamphlet of International Publication No. 02/21204). Further, as means for separating the signal light power from the ASS optical power, there has been known a method using a simplified optical spectrum analyzer. However, the simplified optical spectrum analyzer has a disadvantage in that the monitoring accuracy becomes lower and also an expensive monitor system is needed.
The following problem exists in the above conventional techniques. For example, in the optical amplifier in which the optical amplification medium is managed, such as, the rare-earth element doped fiber amplifier or the concentrated Raman amplifier, it is possible to accurately calculate the noise light power by the known method as described above. However, in the case of a distributed Raman amplifier in which the transmission path fiber is the optical amplification medium, since a fiber parameter of the transmission path fiber is unknown in many cases, there is considered that a predicted fiber parameter value is significantly different from an actual value, or an unexpected loss exists, resulting in a possibility of large error in the calculation value of ASS light.
Specifically, in the case where the ASS optical power is estimated to be larger than the actual value, in the shutdown control described above, since the supply of the pumping light is stopped although the transmission of signal light is able to be performed, the transmission of signal light is suspended. Further, in the output constant control described above, since the signal light is output at the power level higher than the required power, the signal waveform deterioration or the like due to an increase of non-linear effect is resulted, and thus there is a possibility that the system performance is lowered. On the other hand, in the case where the ASS light power is estimated lower than the actual value, in the shutdown control, the pumping light is output although the signal light is in the interrupted state, and in a situation of signal interruption caused by the fiber cutting or the like, there may be a possibility that the pumping light of high power is radiated to outside, to harmfully affect the human body. In the output constant control, since the signal light is output at the power level lower than the required power, the OSNR deterioration is resulted.
Moreover, consideration is made on the case where the wavelength dependence of gain is controlled as described above, for example in the system proposed in Japanese Unexamined Patent Publication No. 2002-72262, a relation between the pumping light power and the signal light output power is expressed by a determinant, and using an inverse matrix of the determinant, the setting of pumping light power is performed so that the required signal light output power can be obtained in each wavelength. However, as in the case of the calculation of ASS light described above, since the fiber parameter of the optical fiber being the optical amplification medium is unknown in many cases, there is a possibility of large error in the setting value of the pumping light power. In addition, in the case where the determinant used for the control does not correspond to an actually laid fiber, it takes a time until the control converges, or the control becomes divergent, resulting in a problem in that the pumping light power is not fixed.
The optical transmission system using the conventional optical amplifier has the following problem, other than the above described problems related to the monitoring of the output power of signal light. Namely, as shown in the description of the shutdown control, since a high power light is output from the optical amplifier, such as the rare-earth element doped fiber amplifier or the Raman amplifier, there is a possibility that the high power light is emitted to the outside air to injure a human body, due to for example, the detachment of an optical connector positioned on an output end of the optical amplifier, the cutting of the optical path connected to an optical output end, or the like.
As a conventional technique for preventing the occurrence of such a situation, there has been known a technique for adding, to the optical amplifier, for example, a function of measuring a reflected return light from an output side optical connector of the optical amplifier and an optical path connected therewith, and based on the measurement result, detecting whether or not an output light from the optical amplifier is emitted to the outside air (refer to Japanese Unexamined Patent Publication No. 9-64446).
An optical connector of a typical physical contact (PC) connection system is deteriorated in the connection performance thereof due to impurities (for example, dust, oil film or the like) attached on a ferrule end surface, or scars on the ferrule end surface. It has been reported that, if a high power light is transmitted through the optical connector which is deteriorated in its connection performance, there occurs the breakage of optical fiber, called a fiber fuse (FF) phenomenon, due to energy convergence by multiple reflection (refer to D. P. Hand et al., “Solitary thermal shock waves and optical damage in optical fibers: the fiber fuse”, Optics Letters, Vol. 13, No. 9, pp. 767 to 769, September 1988, or R. Kashyap et al., “Observation of Catastrophic Self-propelled Self-focusing in Optical Fibers”, Electronics Letters, Vol. 24, No. 1, pp 47 to 49, January 1988)
The above FF phenomenon will be described briefly. For example, as shown in FIG. 41, in the case where impurities or scars are present on an end surface of a ferrule 2001 of an optical connector 2000, a light being propagated through an optical fiber 2002 is diffusively reflected due to the impurities or scars. At this time, if the power of the light diffusively reflected is high, the temperature rise of epoxy resin adhesive 2003 adhering the ferrule 2001 and the optical fiber 2002 becomes higher due to light absorption, leading to an unstable adhesion condition. As a result, the PC connection of the optical connector 2000 becomes unstable, which is one factor causing the FF phenomenon. Accordingly, for the optical connector through which the high power light passes, a particularly careful management of connecting loss becomes necessary.
However, in the conventional optical amplifier disclosed in Japanese Unexamined Patent Publication 9-64446, since the reflected return light of the output signal light, that is, a Fresnel reflected light generated on the connector end surface when the optical connector on the output side is detached, is measured, to detect whether or not the optical connector is detached, there is a problem in that it is impossible to reliably detect up to the optical fiber breakage due to the FF phenomenon which occurs in the optical connector in the insufficient connection state as described above.
Specifically, sometimes the impurities attached on the end surface of the optical connector become absorbers of the light passing through the optical connector. Therefore, there is a possibility that the temperature of optical connector rises due to the light absorption, resulting in the breakage of optical fiber. Since the reflected light is not generated from such absorbers attached on the end surface of the optical connector, in the conventional system utilizing the reflected return light, it is not possible to detect the breakage of optical fiber as described in the above. In the optical connector for when the breakage of optical fiber occurs, since a connecting loss is increased, a desired transmission characteristic cannot be obtained. Further, if the breakage of optical fiber further progresses so that the high power light is emitted to the outside air, there is a possibility of injury to a human body.
Further, the above conventional optical amplifier is constituted to detect the detachment of the output side optical connector, the optical path cutting or the like, based on the measurement result of reflected return light on the output side. Consequently, there is a problem in that it is difficult to cope with the Raman amplifier, in which the pumping light of high power is given from the input side. Namely, for example, in a Raman amplifier 2010 of a configuration shown in FIG. 42, in order to obtain the desired output power, a pumping light Lp having high power of several hundreds mW to several W is output from a pumping light source 2011 to a transmission path fiber 2013 via a WDM coupler 2012. Therefore, it becomes important to supervise a connection state of an input side optical connector 2014. Then, the deterioration of connection state is found, it is necessary to stop or reduce the supply of the pumping light.
However, in the above described conventional optical amplifier, the configuration thereof does not cope with the detachment of input side optical connector, and further, as well as the case of the above described output side optical connector, it is difficult to detect up to the breakage of optical fiber due to the FF phenomenon occurring in the input side optical connector.